1. Field of the Invention
The present invention generally relates to a vehicle driving force control apparatus for a four-wheel drive vehicle. More specifically, the present invention relates to a vehicle driving force control apparatus for a four-wheel drive vehicle that switches between a four-wheel drive state and a two-wheel drive state in response to such factors as a traveling state.
2. Background Information
An example of this kind of four-wheel drive vehicle driving force control apparatus is disclosed in Japanese Laid-Open Patent Publication No. 11-243608. In the vehicle described in that publication, the front wheels are the main drive wheels that are driven by an internal combustion engine, while the rear wheels are the subordinate drive wheels that are driven by an electric motor. When the vehicle is in a four-wheel drive state, both the front and rear wheels are driven together. A clutch and a reduction gear are installed in the torque transfer path between the electric motor and the rear wheel axle. The technology disclosed in this publication employs a drive control method in which the electric motor is rotated in an unloaded state until it reaches a rotational speed equivalent to the rotational speed of the rear wheel axle before connecting the clutch. After the clutch is connected, the output torque of the electric motor is then increased gradually.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved vehicle driving force control apparatus. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.